\subsection{Circuit-Architecture Implications}
\label{subsec:implications}

While recent work has examined the applicability of TFET-based
processors to various domains, the TFET processor models used in these
works lack a detailed consideration of process variation effects at
scales beyond the devices themselves, and propose no response beyond
guardbands. This is a limitation of prior work, as TFETs are only
preferable to FinFETs at lower supply voltages, and process effects
are more keenly felt at these same lower voltages. The net effect of
an accurate consideration of process variation effects in TFETs will
be to shift the cross-over point between where TFET or CMOS based
devices are preferable, as shown in simple model depicted in
Figure~\ref{fig:variabilityshift}.

However, while it is clear that a shift will occur, two key aspects of
the nature of the shift remain unexplored. Firstly, the assumption
that same microarchitecture would be employed across the entire design
space shown in Figure~\ref{fig:variabilityshift} is an
oversimplification.  As the ratio between the fraction of the cycle
devoted to work and guard-banding shifts due to process variation, the
performance-optimal pipeline depth will simultaneously
vary. Similarly, even if the attainable frequency were held constant
by increasing voltage, the optimal pipeline stage complexity, from an
energy efficiency perspective, may also vary simultaneously. We aim to
be the first group to study the co-optimization of $V_{DD}$, frequency, and
pipeline configuration for a TFET based processor under realistic
process variation assumptions. 

\begin{wrapfigure}{r}{0.5\textwidth}
\vspace{-5mm}
\begin{center}
%\centering
% Requires \usepackage{graphicx}
% Requires \usepackage{wrapfig}
\includegraphics[width=0.5\textwidth]{./lib/figures/crossover_variation.png}
\end{center}
%\vspace{-5mm}
\caption{\small For a fixed microarchitecture, applying a variability model shifts the point where TFETs offer superior performance at iso-voltage (From~\cite{Swaminathan_SteepSlope_DAC14}). }\label{fig:variabilityshift}
\end{wrapfigure}

Second, once the above study more clearly describes the crossover
point from FinFETs for variability-aware TFET processor design, we aim
to characterize the portion of the processor design space in which
TFETs can provide advantages over traditional FinFETs. Since, for the
vast majority of applications, parallelism is limited and thermal
constraints apply broadly to given application domains, TFET-based
processors will not always be able to offer the highest performance or
lowest energy for a given application. We will create a taxonomy that
maps applications and their requirements into regions where either
FinFETs, TFETs, or both are plausible implementation choices, and
provides guidance on which would be most suitable, given performance,
energy, or combined metrics of merit.


% LocalWords:  TFET TFETs FinFETs CMOS microarchitecture VDD guardbands
